Earlier Naphtha Hydrotreating (NHT) reactors were designed to operate at very high temperatures of around up to 750° F. Accordingly, the combined feed exchanger was small. However, in recent times, the importance of keeping lower temperatures in the naphtha hydrotreating reactor has been realized. This temperature reduction is performed primarily to reduce the deactivation of the catalyst at high temperatures and secondly to reduce sulfur recombination reactions inside the reactor.
In the existing naphtha hydrotreating processes, the combined feed to the charge heater is heated via indirect heat exchange with the hydrotreating reactor effluent. The combined feed from the combined feed exchanger (CFE) shell side first enters the convection section of the charge heater and then flows to the radiant section. Subsequently, the combined feed is introduced to the NHT reactor and the resulting effluent is sent directly to the CFE for the indirect heat exchange. One problem associated with the revamping of such old naphtha hydrotreating units is that the existing CFE is found to be inadequate for revamps due to the need for more CFE surface area with a large number of CFE shells at the lower reactor outlet temperatures. When the reactor outlet temperature is too low, the existing smaller CFEs are not able to preheat the combined feed to a temperature that fully vaporizes the combined feed. Having two-phase flow through an NHT charge heater, especially a multi-pass heater, can cause maldistribution of the combined feed in the individual heater passes. Further, one or more passes could become liquid full and potentially overheat and lead to a tube failure. Even in single pass heaters it is recommended to have the combined feed fully vaporized inside a fired heater, being well above the dry point inside the tubes. Therefore, the existing smaller CFEs become inadequate to extract enough heat from the reactor effluent to fully vaporize the combined feed to the charge heater.
One of the existing solutions by which this NHT operation can meet revamp requirements is by adding multiple shells in series to the existing CFE. However, there are several drawbacks associated with adding more CFE shells in series. These drawbacks may include extra capital cost of the new exchangers and piping, availability of plot space to install multiple exchangers in series, and finally increased pressure drop in the reactor circuit potentially rendering the recycle compressor inadequate for the revamp.
Accordingly, it is desirable to provide methods and apparatuses to economically provide the additional combined feed preheat required to ensure the existing CFE provide a fully vaporized combined feed stream to the charge heater. Furthermore, other desirable features and characteristics of the present embodiment will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawing and this background.